Histology of Blood Vessels PDF

Histology of Blood Vessels: Introduction to Structure-Function Relationship Sergio Rey–Keim, MD, PhD Ad Astra Assistant Professor of Medicine UCD Conway Institute of Biomolecular & Biomedical Research UCD School of Medicine Vascular histology The cardiovascular system is a transport system: (i.e. ‘p...

Histology of Blood Vessels: Introduction to Structure-Function Relationship Sergio Rey–Keim, MD, PhD Ad Astra Assistant Professor of Medicine UCD Conway Institute of Biomolecular & Biomedical Research UCD School of Medicine Vascular histology The cardiovascular system is a transport system: (i.e. ‘plumbing’ driven by a ‘pump’) Blood vessels contribute to perfusion and homeostasis according to structure, location and functional changes due to vasoactive molecules and the local microenvironment (e.g., pH, CO2, O2, cytokines, etc.) Blood vessel histology Type and diameter Function Elastic ( 1 – 2 cm ) Arteries Muscular ( 0.1 – 1 cm ) Blood distribution at high pressure Arterioles ( < 100 µm ) Flow and resistance regulation Capillaries ( 4 – 30 µm ) O2, nutrient, CO2 and fluid exchange Venules ( 50 – 200 µm ) Capacitance vessels Veins ( 200 µm – 3 cm ) © Dr. Sergio Rey-Keim under CC BY-NC-ND 4.0 To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/4.0/ Vascular differences: artery vs vein Artery Vein Lumen Intima Media Adventitia Figure modified from original design by Assistant Prof. Stuart Bund, UCD School of Medicine Histology of the Tunica Intima Endothelium: single layer of polygonal, flattened epithelial cells, elongated in the direction of blood flow. Basement membrane Sub-endothelium: contains loose connective tissue and fibroblasts. Modified from O’Dowd, Woodford & Young (eds), Wheater’s functional histology (6th Ed); McGraw-Hill Functions of the endothelium in the Tunica Intima Selective permeability barrier Leukocyte adhesion Cytokine and growth factor secretion Modulation of vascular tone upon smooth muscle cells (e.g., release of NO, and prostacyclin [vasodilators] or endothelins [vasoconstrictors]) Hemostasis (von Willebrand Factor, NO, prostacyclin- anticoagulant role when endothelial barrier is intact) O’Dowd, Woodford & Young (eds), Wheater’s functional histology (6th Ed); McGraw-Hill Histology of the Tunica Media Vascular smooth muscle cells: concentric layers, helical arrangement. Elastin fibers: fenestrated sheets Collagen fibers Modified from O’Dowd, Woodford & Young (eds), Wheater’s functional histology (6th Ed); McGraw-Hill Histology of the Tunica Adventitia Collagen and elastin fibers: longitudinal arrangement. Autonomic innervation Lymphatic vessels Vasa vasorum ‘vessels of the vessel’: provide O2 and nutrients to the cells in the vessel wall; more frequent in larger vessels than in arteries. Modified from O’Dowd, Woodford & Young (eds), Wheater’s functional histology (6th Ed); McGraw-Hill Specific vessel types: elastic arteries Media contains alternating layers of smooth muscle cells and elastic laminae Left (H&E) shows SMCs and elastic fibers (E); right, elastin staining (black) Modified from O’Dowd, Woodford & Young (eds), Wheater’s functional histology (6th Ed); McGraw-Hill Specific vessel types: muscular arteries Media contains layers of smooth muscle cells and some elastic fibers Prominent internal elastic lamina (IEL) and less accentuated external elastic lamina (EEL, absent in smaller muscular arteries) M, tunica media A, tunica adventitia Modified from O’Dowd, Woodford & Young (eds), Wheater’s functional histology (6th Ed); McGraw-Hill Specific vessel types: arterioles Media contains 1 – 2 layers of smooth muscle cells Elastic fibers limited to the internal elastic lamina (IEL) No external elastic lamina (EEL) The transition from large to small arterioles is indicated by the loss of IEL Tunica adventitia is very thin E, endothelium M, tunica media O’Dowd, Woodford & Young (eds), Wheater’s functional histology (6th Ed); McGraw-Hill Specific vessel types: venules Capillaries drain into postcapillary venules, similar in structure to capillaries: bearing endothelium and pericytes, larger diameter ≈15–20 µm. Postcapillary venules drain into collecting venules: larger diameter with an increase in pericyte density. Collecting venules drain into muscular venules: media layer with 2–3 layers of smooth muscle cells. Specific vessel types: small and medium veins Tunica media contains a few layers of smooth muscle cells Tunica adventitia contains some elastin and collagen fibers, arranged longitudinally, No elastic laminae M, tunica media; Ad, tunica adventitia O’Dowd, Woodford & Young (eds), Wheater’s functional histology (6th Ed); McGraw-Hill Specific vessel types: large veins Tunica media contains a few layers of smooth muscle cells Tunica adventitia contains some elastin and collagen fibers, arranged longitudinally, No elastic laminae M, tunica media; A, tunica adventitia O’Dowd, Woodford & Young (eds), Wheater’s functional histology (6th Ed); McGraw-Hill Vein structure allows large fluid capacitance Significance of thin wall and wide lumen Easily collapsed and expanded Facilitates function as capacitance (storage) vessels Figure modified from original design by Assistant Prof. Stuart Bund, UCD School of Medicine Structural comparison among arteries and veins Arteries are characterised by: A narrower lumen. A thicker wall and media layer. Greater wall-to-lumen and media-to-lumen ratios. Greater elastin content of the arterial media layer: Especially in elastic arteries (windkessel effect). Dampens pulse pressure. Veins possess valves (intimal projections) that prevent backflow, thus facilitating venous return. © Dr. Sergio Rey-Keim under CC BY-NC-ND 4.0 To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/4.0/ Capillaries A single layer of endothelial cells located upon a basement membrane. Diameter 4 - 10 µm Three forms of capillary classified according to the nature of the endothelium: ❑ Continuous ❑ Fenestrated ❑ Discontinuous (sinusoids) © Dr. Sergio Rey-Keim under CC BY-NC-ND 4.0 To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/4.0/ Continuous capillaries (electron micrograph) Continuous (e.g. skeletal muscle) * * *Tight junctions M, marginal fold P, pericyte (partly surrounds endothelial cell layer, endowed with limited contractile ability; contributes to basement membrane protein secretion) O’Dowd, Woodford & Young (eds), Wheater’s functional histology (6th Ed); McGraw-Hill Fenestrated capillaries Diaphragm may be absent – renal glomerulus EC EC EC Endocrine glands / intestine Fenestrations Figure modified from original design by Assistant Prof. Stuart Bund, UCD School of Medicine Sinusoids Large intercellular gaps Large fenestrations (no diaphragms) Discontinuous basement membrane Diameter ≈30-40 µm: liver, bone marrow, spleen O’Dowd, Woodford & Young (eds), Wheater’s functional histology (6th Ed); McGraw-Hill Summary: structure follows function Arteries: ✓ Thick walled, muscular, high pressure (large, elastic, windkessels) ✓ Regulate pressure and flow (small arteries and arterioles) Veins: ✓ Valves to prevent backflow, thin walls, collapsible ✓ Low pressure capacitance system Capillaries: ✓ Extremely thin walls ✓ Exchange vessels ✓ Maximized transmural diffusion © Dr. Sergio Rey-Keim under CC BY-NC-ND 4.0 To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/4.0/